In quality control and also for monitoring industrial production processes, particularly in the field of precision engineering, optics and in the production engineering of mechanical and electrical microstructures, there is an increasing demand for measurements of work piece surfaces with the highest possible resolution and level of precision.
Thus, for example, DE 10 2008 033 942 B3 discloses a distance sensor operating on the principle of multi-wavelength interferometry, which uses a plurality of laser light sources, the emitted wavelengths of which lie in the optical telecommunication range between 1520 nm and 1630 nm. The signals of the lasers used herein are combined in a common fiber by multiplexers and routed to a multi-wavelength sensor head. In principle, such a multi-wavelength distance measuring method enables interferometric scanning of topologies and surfaces of any objects using reflection geometry, wherein the multi-wavelength method provides a comparatively large, uniquely assignable measurement region and wherein it is moreover possible to achieve measurement accuracy in the nanometer or even in the sub-nanometer range.
Furthermore, DE 60 2004 004 916 T2 has disclosed an optical free-form surface measuring device, in which a contour-scanning distance sensor is placed substantially orthogonally in relation to a surface to be measured. Here, the distance sensor is placed on a rotatable device, which itself is arranged on a platform that can be moved in relation to a measurement frame. Furthermore, a measurement surface is provided on the rotatable device that holds the distance sensor, the distance of which measurement surface to the measurement frame is measured by means of an apparatus for contactless distance measurement. Finally, provision is made for a rotational measurement apparatus for measuring the rotational angle between two measurement directions, wherein the first measurement direction is prescribed by the movement direction between the platform and the measurement frame and the second measurement direction is prescribed by the distance between the distance sensor and the surface of the element to be measured.
In the case of such a sensor, which, for example, scans the surface of an object without contact in one scanning movement, the movement and the accuracy of the positioning of the sensor in relation to the object to be measured play a decisive role.
In order to be able to establish precisely the distance between the distance sensor and the surface to be measured, the sensor must be aligned substantially orthogonally to the surface to be measured and must adapt the alignment thereof in accordance with the contour of the object to be measured. For this adaptation, both translational and rotational movements of the sensor are to be carried out.
Although a translational movement of the sensor in relation to a fixed reference can be established with a sufficiently high accuracy by means of further distance sensors, it is precisely a rotation or tilt of the sensor that was found to be problematic.
In the case of the demanded measurement accuracy in the nanometer or sub-nanometer range, a rotation of the sensor furthermore always also brings about a non-negligible translational displacement of the sensor in relation to the platform carrying the sensor. Thus, the measurement signal from the sensor must be corrected by at least the positional displacement of the sensor caused by the rotational movement of the sensor. The mechanical tolerances of the sensor bearing cause non-reproducible positional changes of the sensor in different angular positions. It is therefore necessary to determine precisely the position of the sensor for each possible alignment of the sensor.